Many types of communication equipment such as radiotelephones employ retractable antennas, i.e., antennas which are extendable and retractable out of the unit's housing. The retractable antennas are electrically connected to a signal processing circuit positioned on an internally disposed printed circuit board. In order to optimally operate, the signal processing circuit and the antenna should be interconnected such that the respective impedances are substantially "matched." Unfortunately, complicating such a matching system, a retractable antenna by its very nature has dynamic components, i.e., components which move or translate with respect to the housing and the printed circuit board, and thus does not generally have a single impedance value. Instead, the retractable antenna typically has different impedance values when in an extended versus a retracted position. Therefore, it is preferred that the impedance matching system alter the antenna's impedance to properly match the terminal's impedance both when the antenna is retracted and extended.
The physical configuration of the switching system and matching network can be further complicated by the miniaturization of the unit and the internally disposed printed circuit board. Many of the more popular handheld telephones are undergoing miniaturization to the point where many of the contemporary models are only 11-12 centimeters in length. Because the printed circuit board is disposed inside the radiotelephone, its size is also shrinking, corresponding to the miniaturization of the portable radiotelephone. Unfortunately, as the printed circuit board decreases in size, the amount of space which is available to support desired operational and performance parameters of the radiotelephone is generally correspondingly reduced. Therefore, it is desirable to efficiently and effectively utilize the limited space in the radiotelephone and on the printed circuit board.
Miniaturization can also create complex mechanical and electrical connections with other components such as the outwardly extending retractable antenna which must generally interconnect with the housing for mechanical support, and, as discussed above, to an impedance matching system operably associated with the printed circuit board in order for the signal to be optimally processed. These retractable antennas generally include a top load element and a rod element.
As is well known to those of skill in the art, retractable antennas typically operate with desired matching circuits, one associated with the extended position and one with the retracted position. In the extended position, the antenna typically operates as a half-wave (.lambda./2) load (the load attributed to the top load element and the rod). In this situation, the associated impedance may rise as high as 600 Ohms. In contrast, in the retracted position, the antenna rod generally operates as a quarter-wave (.lambda./4) load with an impedance typically near 50 Ohms (the load associated with the top load element). Therefore, when the antenna is in the extended position an L-C matching circuit may be needed or desired to match out the additional impedance.
Conventional portable radiotelephones use a variety of antenna connections to switch matching systems into the receive circuit according to the position of the antenna. For example, U.S. Pat. No. 5,374,937 to Tsunckawa et al. proposes downwardly spaced-apart contacts or terminals on the printed circuit board in the radiotelephone housing which act to engage with or short out the associated matching network. Unfortunately and disadvantageously, this type of switching connection can employ a number of lower reliability discrete switching components such as wiping contacts with multiple signal feed points and additionally may use an undesirable amount of space on the printed circuit board. Further, the rod remains in contact with the top load element (such as a helix) and can detune or degrade the operating characteristics of the antenna or can allow noise to be introduced into the telephone through the rod when the antenna is retracted.
One alternative is described in a co-pending patent application, Ser. No. 08/858,982, filed May 20, 1997, and assigned to the assignee of the present application entitled "Radiotelephones with Antenna Matching Switching System Configurations" by Gerard J. Hayes and Howard E. Holshouser. This system employs transversely spaced-apart circuit and antenna contacts to reduce the amount of space on the printed circuit board needed to operate the matching system. An additional alternative is described in a co-pending application, Ser. No. 08/841,193, filed Apr. 29, 1997, entitled "Radiotelephones with Integrated Antenna Matching Systems" by Howard E. Holshouser. Each of these references is hereby incorporated by reference as if recited in full herein.